Here is what I do on occasion..I try to do 500 swings as fast as I can (250L/250R). The fastest I've done this with a 24kg is 16 or 17 minutes. Now, that was breaking up the sets anyway I could. Other times, I might do as many reps as I can in 10 minutes.

I've done 5X50L/50R swings @ 24kg in 18 something....

I'm not sure what's magical about 12 minutes, but it's not a bad idea. I'm also not sure if you are talking about 50 one arm swings (which are a great builder towards snatches) or 2 arm swings.